Electronic flash device

ABSTRACT

The device controls the amount of light emitted by a flash illumination tube by interrupting the energization of the tube during a flash discharge. A main condenser is charged to a predetermined voltage from a suitable source, and is arranged to be discharged through the flash illumination tube when a thyristor, connected in series with the flash illumination tube, is triggered conductive responsive to actuation of a camera element. A discharge preventing condenser is also charged from the source and is arranged to be discharged through a switch element connected in series therewith responsive to a flash preventing trigger signal. When the flash preventing condenser is thus discharged, it triggers the thyristor non-conductive to block discharging of the flash illumination tube. A charging signal trigger circuit and a charging thyristor, having its gate connected to the charging signal trigger circuit, are connected in parallel with the illumination or discharge preventing condenser. The charging signal trigger circuit is activated responsive to discharge of the illumination preventing condenser to trigger the connected thyristor conductive for a rapid recharging of the illumination preventing condenser. Thereby, the delay in the resetting of the circuit for a subsequent flash operation, as in conventional electronic flash devices, is eliminated.

United States Patent 91 Nakamura July 22, 1975 ELECTRONIC FLASH DEVICE [75] Inventor:

Zenzo Nakamura, Urawa, Japan [73] Assignee: Canon Kabushiki Kaisha, Japan [22] Filed: May 9, 1973 g [21] Appl. No.: 358,522

Primary Examiner-R. V. Rolinec Assistant Examiner-Lawrence J. Dahl Attorney, Agent, or Firm-McGlew and Tuttle [57] ABSTRACT The device controls the amount of light emitted by a flash illumination tube by interrupting the energization of the tube during a flash discharge. A main condenser is charged to apredetermined voltage from a suitable source, and is arranged to be discharged through the flash illumination tube when a thyristor, connected in series with the flash illumination tube, is triggered conductive responsive to actuation of a camera element. A discharge preventing condenser is also charged from the source and is arranged to be discharged through a switch element connected in series therewith responsive to a flash preventing trigger signal. When the flash preventing condenser is thus discharged, it triggers the thyristor non-conductive to block discharging of the flash illumination tube. A charging signal-trigger circuit and a charging thyristor, having its gateconnected to the charging signal trigger circuit, are connected in parallel with the illumination or discharge preventing condenser. The charging sig nal trigger circuit is activated responsive to discharge of the illumination preventing condenser to trigger the connected thyristor conductive for a rapid re-charging of the illumination preventing condenser. Thereby, the delay in the resetting of the circuit for a subsequent flash operation, as in conventional electronic flash devices, is eliminated.

3 Claims, 4 Drawing Figures SHEET PATENTEDJUL 22 ms FIG.

FIG. 2

ELECTRONIC FLASH DEVICE FIELD AND BACKGROUND OF THE INVENTION The present invention relates to an electronic flash device, particularly to an electronic flash device suitable as an auxiliary light source for picture taking.

conventionally, there is a known flash device in which booster circuit output voltage of a DC converter, or a high voltage power source, etc. is accumulated at a main capacitor, and the electric charge thereof is discharged through a xenon tube to emit light used as illumination light.

In such a flash device, while discharging is done at a flash discharge tube, it is possible to control the amount of light emitted to an appropriate level considering an object at various distances or illuminated by various peripheral light, by adjusting the amount of the discharge. I-Ieretofore various circuits have been proposed to make the discharge amount to a flash tube vehicle, thereby adjusting the amount of emitted light to a desired value.

As a method to obtain a desired amount of emitted light by stopping the illumination of a flash discharge tube in during discharging of the electric charge of a main capacitor, it is known to provide a by-pass tube in parallel with a discharge tube and to trigger the same during illumination of the discharge tube for shortcircuiting the electric charge of the main capacitor, and it is also known to provide a switching element in series with a flash discharge tube and to trigger the same for shielding the discharging of the discharge tube. Both of these known methods have advantages and disadvantages. That is, in the former light amount control system of the parallel type, while sufficient effect can be obtained with a comparatively simple circuit arrangement, power consumption is great, as the residual electric charge of the main capacitor is short-circuited by the by-pass tube, with the disadvantage that the consumption of the cell is large in a device which employs a small size cell as a power source. Contrary to this, in the latter light amount control system of the series type, as the discharge path is shielded during part of the discharge, the power consumption is smaller than that in the former system, but since a switching element is connected in series in a discharge path of a flash discharge tube, it has the disadvantage that its function apts to become uncertain and the function of the discharge tube at the time of start-up of discharge becomes unstable. Thus each one of the systems has advantages as well as disadvantages.

The present invention employs particularly a light 1 amount control system of the series type to control the illumination time of the flash discharge tube corresponding to a distance from a camera to an object or corresponding to the film sensitivity.

For controlling the illumination time of a flash discharge tube, the discharge current is shielded by converting a switching element, connected in series to a discharge tube, to the non-conductive state.

For suddenly converting the switching element from a conductive state to a non-conductive state, as mentioned above, it is conventional that the charge stored in a condenser connected in a circuit including a fixed resistance is applied to the switching element for converting the latter suddenly to the non-conductive state. As this charging must be done through a fixed resistance, the charging time of the condenser is determined by a time constant which is the product of its capacity and the resistance. Consequently, it is impossible to proceed to the next flash operation until the flashing of a flash discharge tube has been interrupted and the charging of the condenser is completed.

FIG. 1 illustrates a conventional flash illumination device of the series control type. The illustrated device includes a booster circuit 1, of a conventional type, such as a dc. converter, or the like, a main storage condenser 2, and a flash discharge tube 3 having a trigger electrode 3'JThe circuit also includes a silicon control element 4 having a gate 4' and a cathode 4". A trigger circuit 5 is included and is connected to a flash starting signal input terminal 6. A discharge preventing condenser 9 is connected in the circuit in association with fixed resistors 7 and 10 and diodes 8 and 11. A switching element 12 is connected operatively with the discharge preventing condenser 9 and is switched by a flash preventing trigger circuit 13 responsive to a signal provided at a terminal 14. A further fixed resistance 15 is connected in series with trigger circuit 5, and the power source 1 is connected to the circuit upon closure of a power source switch 16.

When switch 16 is closed, main storage condenser 2 and discharge preventing condenser 9 are charged. The trigger signal from flash trigger circuit 5 is applied to trigger electrode 3' of flash discharge tube 3, and to the gate 4' of silicon control element 4, so that the latter becomes conductive and tube 3 emits light. Subsequently, the flash preventing signal from terminal 14 is received by flash preventing trigger circuit 13 and the electronic switching tube 12 is triggered conductive by circuit 13. Thereby, discharge preventing condenser 9 is discharged and a positive potential is applied to the cathode 4' of thyristor 4 to trigger the thyristor nonconductive to interrupt flashing of tube 3.

The purpose of providing diode 8 is to prevent current from flowing through resistance 7 and discharge tube 3 when condenser 9 is discharged, and the purpose of diode 11 is to prevent current from flowing through the circuit of discharge preventing condenser 9, switching element 12, such as a quenching discharge tube or the like, and resistance 10. In the circuit shown in FIG. 1, condenser 9 is discharged with switching element 12 to trigger silicon control element 4 nonconductive. Thus, the time required for the preparation for a following illumination to be completed, after completion of the initial illumination, will be influenced by the charging time of condenser 9, which is charged through resistances 7 and 10.

Therefore, the preparation time from the first illumination to the second illumination will be determined by a time constant factor of the product of the sum of the resistance values of resistances 7 and 10 and the capacity of the condensor 9. Thus, when the illumination preparation time needs to be shortened, the capacity of V the condensor is made small or the resistance value is a diode 8 and switching element 12. Also, when resistance is made small, even when control element 4 becomes non-conductive, the current will flow through the circuit including flash discharge, tube 3, resistance 10 and a diode 11, so that illumination cannot be. interrupted.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS In the Drawings FIG. 1 is an electric circuit diagram of a conventional electronic flash device.

FIG. 2 is an electric circuit diagram of one embodiment of the electronic flash device according to the present invention.

FIG. 3 is a wave form diagram of the function of the circuit shown in FIG. 2.

FIG. 4 is an electric circuit diagram of another embodiment of the electronic flash device according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the first embodiment of the invention shown in FIG. 2, parts corresponding to those shown in FIG. 1 have been designated by the same reference numerals with 100 added. The essential difference between the embodiment of the invention shown in FIG. 2 and the prior art device shown in FIG. 1, is that a charging signal trigger circuit 1 17 and a charging silicon control element 1 18, having its gate terminal connected to a trigger circuit output 117,, are additionally connected in parallel with illumination prevention condenser 109. The circuitry of trigger circuit 117 is such that the behavior of the terminal voltage is detected as condenser 109 is discharged, and a control signal for applying a gate signal to silicon control element 118 is generated.

As the discharge of the flash tube 103 is stopped in the above set-up as the control element 104 becomes non-conductive, the current flows to the circuit containing the flash discharge tube 103, a connecting point 120, the discharge preventing condensor 109, a connecting point 119, and the switching element 112. At this time a positive potential is applied to the connecting point 120 and a negative potential to the connecting point 119. Next, the charging signal trigger circuit 117 is actuated by the positive potential at the connecting point 120, so as to supply a trigger signal to the gate. of the charging silicon control element 1 18.Therefore, the charging silicon control element 118 becomes conductive and the discharge preventing condensor 109 is repidly charged and thereforea positive potential is applied to the connecting point 119 and a negative potential to the connecting point 120. Thus the preparation for the next flashing is completed. The state of charging of discharge preventing condensor 109 shall be explained along with a graph.

In FIG. 3, the ordinate axis shows the charge magnitude, wherein the positive direction represents a case when the positive potential is applied to connecting point 119, and the negative direction illustrates the case when a positive potential is supplied to the connecting point 120. In FIG. 3, when the power source is connected at the point A, charging of condenser 109 increases in accordance with the curve determined by the time constant of the resistance value and the capacity value, and the charge is carried out to a value sufficient to convert discharge preventingsilicon control element 104 from the conductive state to the nonconductive state.

When the flash discharge tube discharges to emit I light, condensor 109 is discharged in accordance with a flash preventing signal at the point C to actuate control element 104 to stop or interrupt flash discharge tube 103 from emitting light. Then, as the first charging of condensor 102 is started through flash discharge tube 103 from the point D and reaches the pointE, the

charging signal trigger circuit is actuated and condensor 109 is re-charged through control element 118. Thereby the direction of the electric charge is reversed and, at the same time, the voltage is elevated instantaneously and the next flash starting preparation is com pleted at the point B. 1

In a flash device with a set-up such that the charging of the condensor 109, having the function to prevent discharging for the light amount control, is made through the resistance 107, another circuit, which con-v tains the silicon control element 118 for charging this i condensor, is provided for stopping the discharge the second time, and so on. By this circuit, the condensor v is reversly charged as the first or initial energization of the flash discharge tube is interrupted, and the charging silicon control element is actuated by the voltage to charge the condensor with a polarity reverse to. its previous polarity, so that it can function to interrupt the discharge of the illumination tube. Thus, the time required before a second flash picture can be taken is decreased, permitting substantially continuous driving of the flash, discharge tube 103. The arrangement is very effective whenit is desired to make a series of flash photographs one after the other.

FIG. 4 shows another example of the present invention. In this figure 201 is a power source cell, for example a low voltage small size dry cell, a Ni-cd battery, etc. 202 is a power source switch. 203 is adirect current voltage booster circuit consisting of,,for example,

a transistor oscillation circuit and a rectification circuit for generating a high voltage directcurrentfrom a low voltage direct current power source. 204 is a main capacitor, and 205 is a transformer inserted in a discharge path of the main capacitor. 206' is a flash discharge tube, 207 is a triggertransforrner forthe discharge tube, and 208 is a condensor for the same. 209 is a synchronizing contact and is closed, forexample, in synchronism with the shutter running by a release button of a camera. 210 is a semi-conductor switching element- (thyristor) connected in series with the flash discharge tube 206. 211 to 223 are elements composing a light amount control circuit for controlling the switching of element 206, wherein 211 is arectifyingdiode, 212 is a smoothing condensor, and 213 is a variable resistance for setting the light amount of the discharge tube at a desired value, 214 is a control switching element which is a programmable uni-junction transistor (PUT). 216

is a condensor of a time constant circuit to determine the control time together with the variable resistance 213, etc. 219 is a thyristor. 217, 218, 220, 221 and 222 are elements composing a circuit to turn off the series thyristor 210, and compose the charging circuit of the commutation condensor 220. 223 is an auxiliary capacitor connected in parallel with the flash discharge tube 206. 224, 225 are the trigger condensor and, resistance, respectively, of the thyrister 210. 226 is a thyristor, 227 is a conductance signal generation circuit having its output terminal connected to the gate of the thyristor 226 and corresponds to the member 117 of FIG. 2.

Next, the function of this device shall be explained. When the power source switch 202 is put ON, the power is supplied to the direct current booster circuit 203 from the battery 201, and direct current high voltage is generated at the output of the circuit 203 by the transistor oscillation circuit and the rectification circuit, whereby the main capacitor 204 is charged. As the charged voltage of the main capacitor 4 reaches a certain value, the oscillation of the booster circuit 203 is stopped by an oscillation control circuit which is not shown in the drawing and the electric charge of the main capacitor will maintain a constant value this state. The condensors 208, 220, 223 at are also charged. At this state, the thyristor 210 and the thyristor 219 and PUT 214 are placed in OFF state. Next, as the synchronizing contact 209 is placed in ON state by a shutter release of a camera, etc., the electric charge of the condensor 208 of the trigger circuit flows to the trigger transformer 207 and a trigger pulse is impressed onto the flash discharge tube 206. At the same time, the thyristor 210 becomes ON by the voltage generated at the resistance 225 and the condensor 224. Then the electric charge of the main capacitor 204 and the auxiliary capacitor 223 is discharged by the flash discharge tube 206, which emits light. At this time of actuating the discharge tube 206, the discharge is made certain by the action of the small capacity auxiliary capacitor 223 which is directly connected in parallel with the dis charge tube. The discharge current from the main capacitor 204 flows to a primary coil of the transformer 205 in a pulse shape, thereby generating pulse wave form output at its secondary coil. This output is rectified by the diode 211 of the control circuit and is smoothed by the condensor 212, and at the same time, with the trigger of the flash discharge tube 206, direct current constant voltage is generated. This voltage works as a power source actuating the control circuit, and the PUT 214 is triggered with the time constant set beforehand at the variable resistance 213, that is, the delay time provided by the condensor 216 and the resistance 213, etc., and the thyristor 219 is switched ON by the terminal voltage of the output load resistance 215 of the PUT. As the thyristor 219 becomes ON, the terminal voltage of the commutation condensor 220, charged with the circuit of the resistance 217, the diode 218, the resistance 221, and the diode 222, is impressed on the thyristor 210 through the thyristor 219 with reverse polarity, whereby the thyristor 210 turns from ON to OFF blocking the discharge path of the flash discharge tube 206. Therefore, the flashing of the discharge tube 206 is stopped and the amount of light emitted is controlled. Here since, the residual electric charge of the main capacitor 204 at this state remains undischarged, the capacitor is charged to a predetermined value with the power from the direct current booster circuit, so that preparation for next flashing is than that in the device of parallel type control system.

While the delay time in the control circuit of this device can be manually adjusted by setting the photographing information of flash photographing at the variable resistance 213, it can be automatically controlled by receiving light reflected from an object.

In the same manner as in the example shown in FIG. 2, after the thyristor 210 becomes non-conductive, the thyristor 226 is triggered conductive through from the gate signal generation circuit 227 and charging current flows to the discharge preventing condensor 220, and the charging of the condensor 220 rapidly proceeds in preparation for the next flashing.

As has been explained above, in the flash device of the present invention a circuit is provided which obviates the difficulties in a conventional flash device having a series type control circuit, that is, the uncertainty in the function at start up and the fact that the control circuit is started at the same time with the trigger of the flash discharge tube, further power consumption is small as a characteristic of a series type control system. Thus it is very effective especially as a small size light control flash device.

What is claimed is:

1. An electronic flash device for a photographic cam era, operable to control the amount of light emitted by a flash tube by interrupting the discharge of a flash tube after initiation of the discharge thereof, said electronic flash device comprising, in combination, a source of electric potential; a first capacitor connected across said source for charging thereby; a flash tube having one terminal connected to one terminal of said first capacitor; a first switching means connected in series to the other terminal of said flash tube, at a junction point, and to the other terminal of said first capacitor, and operable to effectively connect and disconnect said flash tube from said first capacitor; trigger means connected to said first switching means and operable to trigger said first switching means conductive for discharging said first capacitor through said flash tube; impedance means; a second capacitor; said impedance means connecting one end of said second capacitor with one terminal of said source of electric potential; the other end of said second capacitor being connected to said junction point of said first switching means and said flash tube, and to the other terminal of said source of electric potential, for charging of said second capacitor through said impedance means; signal generating means operable to generate a signal for interrupting flashing of said flash tube; first means connecting one end of said second capacitor to a terminal of said source of potential and to said first switching means, and connected to said signal generating means for operation responsive to a flash interrupting signal to be conductive for a predetermined time by the flash interrupting signal so as to reverse the potentials at the opposite ends of said second capacitor and to trigger said first switching means non-conductive; whereby said second capacitor is discharged; and a second means connected in parallel with said impedance means and electrically connected to said first means; said second means being triggered conductive for a predetermined time responsive to discharge of said second capacitor, to form a charging circuit for saidsecond capacitor in parallel with said impedance means and bypassing said impedance means; said second means being triggered conductive after said first means has been triggered conductive and then triggered non-conductive responsive to termination of 3 means triggered conductive responsive to the output of said second capacitor.

3. An electronic flash device, as claimed in claim 2,

in which said second switching means is an SCR. 

1. An electronic flash device for a photographic camera, operable to control the amount of light emitted by a flash tube by interrupting the discharge of a flash tube after initiation of the discharge thereof, said electronic flash device comprising, in combination, a source of electric potential; a first capacitor connected across said source for charging thereby; a flash tube having one terminal connected to one terminal of said first capacitor; a first switching means connected in series to the other terminal of said flash tube, at a junction point, and to the other terminal of said first capacitor, and operable to effectively connect and disconnect said flash tube from said first capacitor; trigger means connected to said first switching means and operable to trigger said first switching means conductive for discharging said first capacitor through said flash tube; impedance means; a second capacitor; said impedance means connecting one end of said second capacitor with one terminal of said source of electric potential; the other end of said second capacitor being connected to said junction point of said first switching means and said flash tube, and to the other terminal of said source of electric potential, for charging of said second capacitor through said impedance means; signal generating means operable to generate a signal for interrupting flashing of said flash tube; first means connecting one end of said second capacitor to a terminal of said source of potential and to said first switching means, and connected to said signal generating means for operation responsive to a flash interrupting signal to be conductive for a predetermined time by the flash interrupting signal so as to reverse the potentials at the opposite ends of said second capacitor and to trigger said first switching means non-conductive; whereby said second capacitor is discharged; and a second means connected in parallel with said impedance means and electrically connected to said first means; said second means being triggered conductive for a predetermined time responsive to discharge of said second capacitor, to form a charging circuit for said second capacitor in parallel with said impedance means and bypassing said impedance means; said second means being triggered conductive after said first means has been triggered conductive and then triggered non-conductive responsive to termination of said flash interrupting signal.
 2. An electronic flash device, as claimed in claim 1, in which said second means is a second switching means triggered conductive responsive to the output of said second capacitor.
 3. An electronic flash device, as claimed in claim 2, in which said second switching means is an SCR. 